A secret lab (owned by a private company) near Chicago creates a virus strain called 'ZB01' (to be sold as a biological weapon). There is a small town nearby and, unannounced to the residence there, the lab has injected ZB01 into their water supply (to test its effects), and turning the entire town into zombies.

These are your stereotypical zombies. Get infected and you mutate into a zombie. They hunger for human flesh and are attracted mostly by high pitched or humanlike sounds.

What are the physical requirements of the zombies in order for them to not be immediately killed off?

What would the human-to-zombie timetable need to look be (Approximately)?

Approximately how many initial zombies (town's population) would it take to infect the country with those attributes?

Given the current tech level of Earth, is it feasible for zombies to take over even with their physicality?

$\begingroup$@Samuel very true x) it just frustrates me so much when zombie movies completely disregard the initial outbreak!$\endgroup$
– wposeyjrMay 8 '15 at 16:52

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$\begingroup$Did the lab report the effects immediately, or perhaps try to cover it up? The response time of forces with the ability to control such an outbreak would probably have significant impact on whether the infected ever even make it out of their own town or the local area.$\endgroup$
– MichaelMay 8 '15 at 16:57

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$\begingroup$@Michael the lab is run by a highly classified team of unregulated US government scientist who have absolutely NO oversight what-so-ever. After discovering the 'zombification', the lab wiped everything and moved out to remain undiscovered.$\endgroup$
– wposeyjrMay 8 '15 at 16:59

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$\begingroup$@DJMethaneMan No, that's not at all realistic. Slightly more feasible would be releasing it on a foreign population, but still not realistic.$\endgroup$
– SamuelMay 8 '15 at 17:47

8 Answers
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Let's say ZB01 was supposed to be a super-soldier serum, but it has gone horribly wrong. The component meant to give the super soldier an indomitable will and unerring ability to follow commands backfired when they gave it to soldiers on an empty stomach. Hunger, one of the most basic and primal drives from the reptilian brain became the single command the soldiers would follow. When it was given to the town through the water supply, they became strong and had an unquenchable thirst... for blood (cackle, cackle, lightning and thunder). The super-soldier serum has given them excellent strength, but burns energy so quickly that they remain always hungry. The hunger screams in their minds, driving them mad.

What are the physical requirements of the zombies in order for them to
not be immediately killed off?

While it's not clear what they are facing which may or may not immediately kill them off depending on physical characteristics, they should be at least as capable as humans. Humans with no thought of self-preservation, response to pain, and an unerring will to feed.

What would the human-to-zombie timetable need to look be
(Approximately)?

This is actually incredibly important if this affliction is to spread around the world. People need to be able to get on planes and fly around the world. The TSA does not admit zombies on to planes. The transformation should be dormant for at least a week and that's when the hunger starts.

Approximately how many initial zombies (town's population) would it
take to infect the country with those attributes?

A small number will do if the population quickly spreads out across the country while infected but unchanged. This would be easily achieved in a place like Las Vegas or New Orleans during Mardi Gras. The infected and usually wasted people would fly home after the weekend and begin infection epicenters all over the world.

Given the current tech level of Earth, is it feasible for zombies to
take over even with their physicality?

Zombies are a fire on the human race. They burn through fast and hot, destroying the humans that catch, but they burn too quickly. They won't take over. They might do a whole lot of damage and the world will never be the same. Pockets of humanity will remain. Bunkers, remote places, astronauts on the ISS, humans will remain and repopulate.

$\begingroup$Not to self: When releasing my zombie virus be sure to hit New Orleans in Mardi Gras.$\endgroup$
– JDSweetBeatMay 8 '15 at 17:49

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$\begingroup$+1 for the last line. Also, I think that if the incubation period for the virus was that long then symptoms would have to be masked until the very last second. If people figure out the warning signs they'll be able to kill people pre-zombification!$\endgroup$
– wposeyjrMay 8 '15 at 17:51

The only way it's possible is if the trigger for the zombie is airborn and converts a substantial percentage of the population on infection.

Otherwise an organised military force will obliterate any number of zombies. It's just not even slightly a threat. Zombies cannot cope with structural defences, armoured vehicles, firearms, or even very basic tactics.

Essentially the infection needs to strike hard, and convert a massive percentage of the populace - a high enough percentage that the military is decimated. We're talking 99.9% zombification rates from infection by a highly contagious airborn pathogen that manages to spread worldwide before it is detected.

This changes of course if you add super-powered or intelligent zombies, but then they are not really zombies any more.

$\begingroup$Well... if the virus is airborne, it might infect the soldiers, thus creating the next group. Might be interesting to have a moderately large number of zombies, armed with assault rifles and tanks...$\endgroup$
– BurkiMay 12 '15 at 10:50

1. Physically, to survive against the modern forces they would no doubt face eventually and to survive in general, they would need to be hardy. Humans are actually not terribly durable, and rotting ones even less so. You have to think that over time they're going to face the seasons, if their bodies can't hold up to heat/cold/weather, then their spread is going to be not insignificantly limited by that.
They would also need to be fast enough and strong enough to catch people. While that may not be too difficult initially when no one knows they exist, once people know to watch for them good luck catching them. Locked doors are probably durable enough to stop any zombie without above-average strength, and cars and other vehicles are going to be able to outrun them, probably even relatively healthy adults too unless they have above average speed.
In addition, in my opinion, they need to retain some level of intelligence. Humans got to where they are because of their ability to use tools, be creative, and plan ahead. If these are just your typical mindless shamblers, bait them into an alley somewhere and box them in. Dispose of as you see fit (fire, explosives, bullet, drop a brick on them, w/e), rinse and repeat.

In addition, lone zombies are rarely scary and probably not an issue unless they're some kind of super mutant. So you'd probably want them to have some sort of pack mentality. If they have lower levels of intelligence this makes sense anyway, we know many animals work in packs/prides/families to increase their chance of survival, so it stands to reason that any creature with baser instincts is likely to come to a similar conclusion.

So in my opinion there would need to be something that enhances their speed/strength and they need to either retain or mimic some form of intelligence. They should also probably roam in groups for practical reasons.

2. Assuming, as you said in the comments, that the incident isn't reported then I don't think the time it takes to go from bitten to infected is terribly important so long as it happens before word gets out. After that, presumably your zombies will disperse in all directions in search of food. Initially they'd likely happen across the slow, the weak, and the isolated making easy targets. Some may die as they encounter larger cities, but enough of them would probably make it into other areas that their numbers would grow.

3. This largely depends on the attributes of the zombies, as mentioned in one. The hardier/faster/stronger/more relentless the zombies are, and the harder to kill, the fewer you need of course. To give any kind of actual number is beyond any reasoning I can come up with.

4. I'm going to have to go with no unless your zombies are super human and capable of using tools. Even if they manage to deal with the various armies, they're only on one continent. If, let's say since you started in Chicago, all of America was infected. Unless your infection time is long enough for a flight/boat trip across the oceans, you're not going to expand beyond the americas with your zombies. That being the case, the other countries on the other continents can either choose to simply never go there again, or they could bomb it to obliteration, or just pick a point to start from and systematically destroy everything. With such a focused effort, unless your zombies are driving tanks and using weapons, eventually they're going to be overcome by any kind of organized armed force.

I would recommend a Typhoid Mary (carrier) sort of situation, if it was me. This is likely best done as a multiple dispersal vector virus, or a single dispersal vector but not as a direct bite situation. Especially as you want the original infection to be introduced via the water supply, and water can carry viruses rather easily:

Viruses are a major cause of human waterborne and water-related diseases.

A percentage of the population carries the zombie infection without turning. They 'escape', or are 'rescued', either by dumb luck or they don't 'smell' human to the turned zombies, making them not interesting as food. Alternately, they are clear of the origin zone before anyone realizes there's a zombie infection, so they have no idea they're infected with something and spreading it. Either way, they reach thus far uninfected portions of the population and disperse the infection like any other virus: surface contact, interpersonal contact, bodily fluids, sex, tears, sneezing, whatever. The uninfected don't realize the carrier is a carrier until it's too late.

If you're rooting for the zombies, there's no clear indication from blood or genetic material or whatever whether someone is a carrier or purely uninfected, and only discernible evidence of infection is that occurring during/after the turn. Also, shooting the zombies, and even setting them on fire, increases the risk of blood-borne infection spraying everywhere, or in the case of fire, not wholly burned particles containing the virus becoming airborne. This could potentially cause a bigger problem than it would solve. Breathing zombie smoke or blood spray, or even skin absorption, makes this war even trickier.

Bear in mind that zombies aren't necessarily dead, though this is a popular choice. Alternately, a colony of zombie virus might animate a person or possibly an animal by combining with the individual cells, if it reproduces rapidly enough and gets a good hold. There's also various parasites and forms of venom which can cause mind control effects in their targets, and the zombie virus could act similarly:

Zombies may still be a thing of fiction, but some parasites more or less turn their hosts into the walking dead. These masters of mind control manipulate their hosts from within, causing them to act in self-destructive ways that ultimately benefit the parasite.

This isn't to say that these aren't stereotypical zombies, only that the methodology of zombification isn't necessarily death, but easily some form of biological manipulation of one kind or another.

In other words:

What are the physical requirements of the zombies in order for them to not be immediately killed off?

These would be the biological requirements, such as ability to be detected before turning, ability to transmit without being infected (carriers) rather than the purely physical (strength, speed, intelligence, etc.)

What would the human-to-zombie timetable need to look be (Approximately)?

This is a complicated issue, but I'd say probably anywhere from 6-48 hours, with carriers. Possibly determined by different people's biological and/or genetic makeup. Differences in infection time would make it more complex to pinpoint the carriers or have a distinct idea how long one has.

Approximately how many initial zombies (town's population) would it take to infect the country with those attributes?

I'd call it around 10% of the infected being carriers, too few and fewer would spread to other locations, too many and the carriers' existence would be noticed much more quickly.

Given the current tech level of Earth, is it feasible for zombies to take over even with their physicality?

With carriers, it could be, if the carriers did not show indications and neither did the infected until the turn itself began, however long that took. Bite marks are clear indications, so to increase the odds of the zombies outnumbering (I hesitate to say taking over, since they really just want food, right?) the uninfected the infection vector would have to be more viral in nature than violent, though of course we would also expect the turned to start eating people just because it's what they do.

This is my first answer submitted on Stack Exchange, I hope I did it correctly and that it may provide you with some useful ideas.

A zombie can not go up against modern humans and win in a one-on-one fight. Machine guns beat stupid creatures, even a hundred stupid creatures are beaten by a machine gun. Thus, if you have less then 100 to 1 in your number of zombies to military well, the zombies are going to loose.

Because such a small percentage of zombies will reach a human to attack and bite him there will be only a small number of zombies produced from each bite, 1 out of every 100 zombies makes a new zombie, your second generation zombies are 1% as many as your first, and your third generation of zombies are .01% as strong as your first, this is pretty rapid dimension returns.

Making zombies able to sustain a few bullet strikes, or fast, is not enough. Intellect and modern weapons just give humans too great an advantage. Sure if the zombies manage to reach a civilian area they may be able to spread faster...but even civilians have guns, one hunger with a rifle will still take out a few zombies with him. Your zombies may be able to grow by about 50% going up against your average civilian population, when you factor in deaths to civilian guns, and civilians barricading themselves or escaping. After all if I simply hope in my car and start driving I can escape classic zombies quite easily...

This is ignoring the ability to use firebombs and air attacks on any massed zombies without any risk at all. the ability to evacuate an area to remove any 'food' and let the zombies die out on their own etc etc. Realistically a zombie can't be much more durable then humans, and they would still potentially starve or degrade rapidly over time if at all realistic.

Thus the most plausible scenario for a zombie Apocalypse is to translate most people to zombies in one go. A virus spreads and kills most of the world population, producing a massive number of zombies. Even if we can kill 100 zombies for every one human we still don't stand a chance if there are 1,000 zombies per one human...

Since this is unlike your scenario there are some things that you can do to try to make it a little closer...

1) make the virus spread from something other then zombie bites. Maybe the virus spreads the same way any virus would spread, through the air or physical contact with infected non-zombies, and most new zombies come about from accidental infection from a non-zombie infected, not from zombie bites. Alternatively, have the zombies explode spores of virus that spread rapidly when they die, so even killing a zombie won't prevent you from potentially being infected. Though explosive infective zombies isn't enough to contract an organized military effort to stop them unless they still have a huge number advantage.

2) infect creatures other then humans! This gives two useful advantages. First, it makes far more zombies, since human zombies will have a much easier time preying on wildlife, converting a huge percentage of that wildlife. Now in addition to human zombies you have a huge number of zombie dogs increasing the threat to humans and spreading zombiesm. More importantly, if you infect small creatures you have a new avenue for spreading zombies. Perhaps zombie rats are one of the largest avenues to zombiesm spreading. A single zombie could infect a huge number of rats, one dead body feasted on by rats would infect dozens, and the zom-rats could effect other rats quickly. These rats could then quickly swarm around biting humans. Because they are small and don't run in packs there is a less obvious avenue for military with big guns to attack. The approach for defending against them is different and more logistical the militaristic. If you allow bugs like mosquito and/or fleas to be infected...well humanity is pretty screwed at that point. If you couple this with a long delay from when someone is infected to when they become a zombie you can allow a huge portion of the population to be converted before we know there is a problem. Maybe say the more of the infection you get the faster you convert, so a bite from misquote will take months to convert you, a bite from a human zombie only days...

3) make goverments slow to respond to the attack. Perhaps people think that zombies can be saved, and are thus unwilling to unleash lead-death upon them at first. Maybe politics slow the response time, or even people refuse to realize it happened.

Another thing to consider is that zombies can't cross large body of water. If zombies are unleashed in a single city then only one land mass (the Americas or the 'old world' depending on where it starts) is in danger. Unless you go with the approach of having the virus lay dormant for a long time, long enough for people to travel to other worlds and spread it before were aware of the danger, then humanity as a whole isn't in danger.

I heard a quote on CBC from a researcher who said there is no way zombies as popularly portrayed in media could survive more than a week or so, but I can't remember when I heard it, so I can't give a direct quote. Zombies are always represented as being dead and decaying, yet somehow continuing to live. The researcher said, basically, that there is so much decomposition happening in nature that any dead biological matter would be eaten by bacteria, insects, small animals, etc. within a few weeks. The idea that a partially decomposed but somehow living upper body severed at the waist could continue to live in a forest for months after dying and attack someone who came upon it is patently ridiculous. Millipedes, my friends, millipedes will be the end of any zombie apocalypse. I often think about this while watching The Walking Dead. Nothing can be both alive and dead at the same time. Either zombies are alive, with the processes of circulation, breathing, digestion that keep living organisms alive, or they are not, at which point they immediately become food for other organisms. Scientifically, we can't have it both ways, as psychologically satisfying as the concept of the undead is. That's my buzzkill answer.

$\begingroup$So according to this article (as well as mainstream science as of late) "viruses today are thought of as being in a gray area between living and nonliving: they cannot replicate on their own but can do so in truly living cells and can also affect the behavior of their hosts profoundly."$\endgroup$
– wposeyjrMay 13 '15 at 14:34

$\begingroup$I'd agree that zombies would have to largely functional but there is no reason the outside flesh could not be non-functional. Millipedes kept at bay by the actual infection.$\endgroup$
– user2617804May 14 '15 at 6:29

An interesting take I've seen on the zombie apocalypse was in the Last Blood webcomic (which, unfortunately, stopped updating mid-story in 2010). In Last Blood, a small band of human survivors is protected by a group of vampires for their mutual survival. The vampires need the humans' blood for food, and the humans need the physically superior vampires to protect then from zombie attacks. The zombie contagion turns out to be...

...a modified/mutated version of vampirism.

For a month or so, zombies were more along the shambling, slow, stupid variety. But once a zombie had been a zombie long enough, they suddenly gained super-speed, super-strength, and some level of intelligence or group intelligence...I don't recall the details.

So, take a situation where zombies become more powerful/dangerous/infectious the longer they've been zombies. In a situation like this, a small group of zombies (eg a small town) that goes unnoticed long enough would become the super-powered core that drives the larger, weaker zombie horde's growth.

Just when it seems like the humans are getting the upper hand and will survive, the zombie horde becomes super-powered and the tables turn.

Just a partial answer, but if you want the initial outbreak to spread, the location of the "small town" is at least as important as the population. Since the lab is near Chicago, would Rosemont be a valid choice? It's population is only a little over 4000, but it's a huge commercial center. There's the Allstate Arena, the convention center, the outlet mall, and a lot of big hotels, offices, and other businesses. Oh, and it's right next to the busiest airport in the world. Of course, this might be cheating on the meaning of "small town," but since most of the people in Rosemont are there for business or entertainment reasons and live elsewhere, this will help the infection spread across the Chicago area, and the country.